The purpose of this project is to assess the relative contributions of genetic and environmental factors to alcohol drinking and response to alcohol challenge. The project is composed of three studies. In the first, male monozygotic (MZ) and dizygotic (DZ) twin pairs complete a 28-day record of their alcohol intake. These data will permit estimation of the relative contributions of genetic and environmental factors to frequency, amount, and pattern of alcohol consumption. In addition, self-reports of contact between cotwins will permit estimation of the importance of shared environment on cotwin similarities in alcohol drinking. In the second study, male MZ and DZ twin pairs receive, in three separate testing sessions, a placebo and two doses of alcohol (0.40 and 0.80 g/kg of 95% ethyl alcohol). The protocol consists of electrophysiological measures (e.g., brainstem auditory evoked responses, spectrally analyzed EEG, and visual and auditory event- related potentials), a behavioral measure of attention, self- reports of affect, and a measure of standing stability. The use of placebo and multiple doses of alcohol will permit conclusions about the effects of alcohol on information processing, response production, mood, and motor activity. The twin design will provide information on the relative contributions of genetic and environmental factors to variability in these measures in the drug- free state and following response to alcohol challenge. In a third study, nontwin males are tested in two separate sessions using a protocol identical to that employed in the twin study. This study will permit estimation of the retest stability of the alcohol effects. This information will be critical in the dose- response analysis and in the genetic analyses of the twin data.